Critics point out the extension offers only a temporary reprieve for the UK and should serve as a "big wake-up call" to Boris Johnson, the London mayor, and the government to take further steps to clean up London's air.

The department for environment, food and rural affairs welcomed the decision and conceded that "more needs to be done" to improve air quality in light of its impact on public health.

But Johnson insisted that the range of measures already outlined in his air quality strategy would ensure the UK averted the long-term risk of "crazy" fines from Europe.

The commission's environment commissioner, Janez Potocnik, issued his long-awaited decision on Friday on the UK's request for an extension to meet the 2005 directive on limit values for PM10s – dangerous airborne particles that contribute to thousands of premature deaths.

Potocnik said he had agreed to an extension until 11 June because the government had successfully demonstrated that the UK would reach compliance by then.

But because this was projected to be met within a "very narrow margin", the extension was agreed on the condition that the London air quality plan is revised by June and submitted to the commission for scrutiny by November this year.

Potocnik said short-term measures will have to be introduced to control "or, where necessary, suspend activities" that contribute to the risk of the limit values being exceeded.

"Air pollution from PM10 has serious impacts on human health," Potocnik said. "That is why EU legislation sets strict standards. The commission expects member states to clearly demonstrate that they are doing their utmost, in the interests of their citizens, to comply with the standards in the shortest possible time."

The decision states that the government's re-submission "satisfies the conditions set in the directive", with the caveat of short-term measures being put in place.

"It is up to the UK to decide which specific actions to introduce," the commission said. "These could include measures in relation to traffic, construction work, ships at berth and the use of industrial plants or products and domestic heating. London's air quality plan will need to be revised by 11 June 2011 to include the short-term measures and submitted to the commission."

Simon Birkett, the founder and director of Clean Air in London, said the European commission's "temporary and conditional time extension" would serve as an ideal mechanism to "maximise pressure on the UK to comply with health-based laws".

Poor air quality is seen as a major public health issue. A report by the House of Commons environmental audit committee concluded last year that it made asthma worse, exacerbated heart disease and respiratory illness, and "probably causes more mortality and morbidity than passive smoking, road traffic accidents or obesity".

Figures published last year by city hall revealed 4,300 premature deaths were caused by poor air quality each year, costing £2 billion a year.

Johnson was heavily criticised by opposition parties and campaigners for unravelling environment policies put in place by his predecessor, Ken Livingstone, such as his decision to delay by 15 months the introduction of phase three of the Low Emission Zone (Lez), which fines the most polluting vans, and the scrapping of the western extension of the congestion charge zone last Christmas.

Birkett said: "The mayor has stumbled and fallen after backward steps on air quality. This is a big wake-up call to the mayor and the so-called 'greenest government' in the run-up to the London 2012 Olympics."

Friends of the Earth's London Campaigner Jenny Bates said London's air pollution was a "scandal" and urged the mayor to come up with an urgent plan to protect Londoner's health.

Defra, which sent the submission on behalf of the government, welcomed the commission's decision, which would be considered "in detail".

A spokesman said: "The acceptance of our case reflects well on the strong efforts made by the mayor, local authorities and the government to put measures in place to improve air quality to benefit public health. But more needs to be done to improve air quality given the strength of evidence around the damaging health impacts of air pollution."

But Johnson went on LBC radio on Friday to make clear his mayoralty would have "nothing new" to put in the plan over and above the measures outlined in his strategy.

City hall claims the commission was not able to formally consider the full range of measures being taken by the mayor to tackle pollution hotspots in the capital because many were not included in the UK submission.

Johnson said that his team had gone to Brussels to spell out first-hand measures that had been omitted, such as the use of dust suppressants in pollution hotspots and redeploying the cleanest buses in the worst pollution areas of London, which will now be officially submitted. "I don't think we are going to have to introduce so called emergency measures such as road closures and that kind of thing," Johnson said.

He insisted on Friday that the package of measures presented to the commission was "powerful and convincing" and would "obviate the crazy £300m fine from Brussels".

In a sideswipe to the government, Johnson told LBC: "The lesson is when a great city like London is faced with fines of this kind it's absolutely vital that we have a direct dialogue with Brussels. We have got to be negotiating directly with the commission and explain what we are doing."

The commission began legal proceedings against the UK in January 2009 for failing to meet air quality standards that came into effect at the end of 2004.

The UK government turned down a first request for an exemption in December 2009 on the grounds that insufficient information had been provided to allow the commission to assess whether the limit values for PM10s could be met by the deadline.

Just days before losing power, the Labour government reapplied for a time extension.

The commission's decision gives the UK only breathing space, since the previous written warnings are still in place. The government could still end up to court if it fails to reduce pollution this year in a fast-track process that could lead to multimillion pound fines.

Darren Johnson, a Green party member of the London assembly, said the "unprecedented step" of requiring a set of emergency plans to be drawn up suggested the commission "simply don't trust the UK government and mayor to protect the health of Londoners".